STORY HIGHLIGHTS

House of Representatives speaker Paul Ryan and senator John McCain had both criticised Trump's feud with Captain Humayun Khan's family

US Republican presidential nominee Donald Trump ratcheted up tensions within his party on Tuesday by denying two leading figures, House of Representatives speaker Paul Ryan and senator John McCain, support in their re-election bids.

Trump told The Washington Post in an interview that he could endorse neither Ryan, the top US elected Republican, nor McCain, senator from Arizona and a former Republican presidential nominee, as they face challenges in their states' primary contests ahead of the November 8 general election.

Both Ryan and McCain had criticised Trump's feud with the family of army Captain Humayun Khan, who died in the line of duty in Iraq in 2004 and was awarded the Bronze Star Medal for bravery after his death.

The discord comes just two weeks after the Republican National Convention in Cleveland that formally nominated Trump in the presidential race.

It is the latest rift in a party already frayed by internal dissent over its standard bearer, seen in stark relief at the convention where McCain was among high-level party members who essentially snubbed Trump by choosing not to attend. Mitt Romney, the 2012 Republican nominee, and former presidents George HW Bush and George W Bush also did not attend the convention.

Trump has had a running dispute with Khizr and Ghazala Khan since they took the stage at last week's Democratic convention to cite their son's sacrifice and criticise Trump's proposed ban on Muslims entering the United States.

The uproar has led many Republicans to distance themselves from Trump and voice support for the Khan family.

Trump, mirroring the language Ryan used about supporting the nominee before his eventual endorsement, told the newspaper he was "not quite there yet" on endorsing Ryan in next Tuesday's Wisconsin primary, and that he had "never been there" with McCain, who will be on the ballot in primary elections in Arizona later this month.

McCain had a "very friendly" meeting with Trump's vice presidential running mate, Mike Pence, on Tuesday in Arizona, where Pence was visiting, a McCain spokeswoman said.

Trump said Ryan had sought his endorsement, but that as of now he is only "giving it very serious consideration".

"And we are confident in a victory next week regardless," campaign spokesman Zack Roday said in a statement.

Ryan is favored to win against primary challenger Paul Nehlen, who Trump praised as running "a very good campaign". In a mid-July survey by Harper Polling, Ryan was ahead of Nehlen by nearly 50 points.

Trump has troubled many in the Republican establishment with his off-the-cuff, often insulting style, and controversial policies, including the proposed ban on Muslims and his plan to build a wall along the Mexican border to keep out illegal immigrants.

President Barack Obama on Tuesday blasted Trump as unfit to be a President and questioned why any Republican would support the New York businessman, who is seeking his first public office.

"The question I think that they have to ask themselves is, if you are repeatedly having to say in very strong terms that what he has said is unacceptable: Why are you still endorsing him?" Obama, a Democrat, said at a White House news conference with Singapore's prime minister, Lee Hsien Loong.

Congressman abandons Trump

The chorus of complaints against Trump grew on Tuesday.

Representative Richard Hanna of New York became the first Republican in Congress to endorse Democratic nominee Hillary, although several other Republicans in Congress have said they will not support Trump.

Hanna, who is retiring from the House of Representatives rather than seek re-election, said his decision was prompted by Trump's attacks on the Khan family. He called Trump "deeply flawed in endless ways," "unrepentant" and "self-involved."

Opinion polls put Clinton in the lead

Trump has fallen behind Clinton in opinion polls made public since the parties held their nominating conventions last month.

Clinton extended her lead over Trump to 8 percentage points, according to a Reuters/Ipsos poll released on Tuesday, from 6 points on Friday. About 43 per cent of likely voters favour Clinton, 35 per cent favour Trump, and 9 per cent picked "other."

Trump also has trailed Clinton in fundraising. Clinton reported raising nearly $90 million in July for her campaign and the Democratic Party, with more than half the donations coming from new donors.